As a paper feed table for use in various image forming devices such as printing devices, duplicating devices and facsimile devices, there is known the elevator type paper feed table device provided with a paper feed table intended for placing a stack of paper thereon and adapted to be vertically moved by an electric motor.
In an elevator type paper feed table device, it is detected by using upper limit detecting means when the upper surface of the paper stack on the paper feed table has been positioned by the vertical movement of the paper feed table at a prescribed height for taking out the uppermost paper sheet with paper feed rollers or the like, and once it is detected that the upper surface of the paper stack on the paper feed table has reached the prescribed height, the electric motor is stopped from lifting the paper feed table. Every time it is detected by the upper limit detecting means that the upper surface of the paper stack on the paper feed table has gone below the prescribed height by more than a prescribed amount due to the feeding of the paper from the paper feed table, the electric motor is activated so as to lift the paper feed table by the corresponding amount.
In such an elevator type paper feed table device, it is essential for accurate and satisfactory paper feeding action to be carried out at an appropriate paper feed pressure that the paper feed table stops its upward movement as soon as the upper limit detecting means has detected that the upper surface of the paper stack on the paper feed table has reached the prescribed height. If there is any fluctuation in the height of the paper feed table, the paper feed pressure also fluctuates, and a satisfactory sheet by sheet paper feeding action cannot be attained particularly when relatively thin paper is to be fed.
Based on such a consideration, it is conceivable to reduce the rotational speed of the electric motor by controlling the voltage of the electric power supplied to the motor when the thickness of the paper to be fed is relatively small so that the speed of the vertical movement of the paper feed table may be reduced and the unpredictable overrunning of the electric motor due to the inertia of the electric motor as it stops may be controlled.
However, according to this proposal, the setting must be changed depending on the thickness of the paper, and if the setting is not appropriate, the situation may even get worse.
When the voltage of the electric motor is controlled so as to reduce the rotational speed of the electric motor, the resulting reduction in the output torque of the electric motor may cause a failure to properly lift the paper feed table or may even cause a complete seizure or stoppage of the electric motor.
When new paper is added or when paper jam is removed, the amount of the lift of the paper feed table required for positioning the upper surface of the paper stack on the paper feed table to the height that can be detected by the upper limit detecting means increases as compared to the normal lift of the paper feed table when paper is fed sheet by sheet. At such a time also, if the rotational speed of the electric motor is restricted, and the speed of the lifting of the paper feed table is reduced, a substantial time period or a substantial waiting time period is required before the paper feed table is lifted to the height at which the upper surface of the paper stack on the paper feed table is detected by the upper limit detecting means.